The present invention relates to a process for the enantioselective enzymatic reduction of a keto compound to the corresponding chiral hydroxy compound, wherein the keto compound is reduced with an oxidoreductase. Furthermore, the invention relates to new oxidoreductases for use in the enantioselective reduction of keto compounds to chiral hydroxy compounds.
Optically active hydroxy compounds are valuable chirons with broad applicability for the synthesis of pharmacologically active compounds, aromatic substances, pheromones, agricultural chemicals and enzyme inhibitors. Thereby, an increasing demand for chiral compounds and thus chiral synthesis technologies can be noted particularly in the pharmaceutical industry, since, in the future, racemic compounds will hardly be used as pharmaceutical preparations.
The asymmetric reduction of prochiral keto compounds is a sector of stereoselective catalysis, wherein biocatalysis constitutes a powerful competitive technology versus chemical catalysis. The chemical asymmetric hydrogenation requires the use of highly toxic and environmentally harmful heavy metal catalysts, of extreme and thus energy-intensive reaction conditions as well as large amounts of organic solvents. Furthermore, these methods are often characterized by side reactions and insufficient enantiomeric excesses.
In nature, reductions of prochiral keto compounds to hydroxy compounds and vice versa occur in numerous biochemical pathways, both in the primary metabolism and in the secondary metabolism, in every organism and are catalyzed by different types of secondary alcohol dehydrogenases and oxidoreductases. Normally, these enzymes are cofactor-dependent.
The basic feasibility of using biocatalysts for the reduction of prochiral keto compounds to chiral hydroxy compounds was repeatedly demonstrated in the past on the basis of model systems, wherein both isolated oxidoreductases and various whole-cell biotransformation systems were used for the task. Thereby, the biocatalytic approach turned out to be advantageous essentially with regard to mild reaction conditions, lack of byproducts and often significantly better achievable enantiomeric excesses. The use of isolated enzymes is advantageous over methods involving whole cells with regard to the achievable enantiomeric excess, the formation of degradation products and byproducts as well as with regard to the product isolation. Moreover, the use of whole-cell processes is not possible for every chemical company, since specific equipment and know-how is required therefor.
Recently, it has been possible to demonstrate that the use of isolated oxidoreductases in aqueous/organic two-phase systems with organic solvents is extremely efficient, economical and feasible also at high concentrations (>5%). In the described systems, the keto compound to be reduced, which usually is poorly soluble in water, thereby forms the organic phase together with the organic solvent. Also, the organic solvent itself can partly be dispensed with. In that case, the organic phase is formed from the keto compound to be reduced (DE 10119274, DE 10327454.4, DE 103 37 401.9, DE 103 00 335.5). Coenzyme regeneration is thereby achieved by the concurrent oxidation of secondary alcohols, for which, in most cases, the inexpensive water-miscible 2-propanol is used.
Examples of suitable R- and S-specific oxidoreductases and dehydrogenases of high enantioselectivity are: Carbonyl reductase from Candida parapsilosis (CPCR) (U.S. Pat. No. 5,523,223 and U.S. Pat. No. 5,763,236, (Enzyme Microb Technol. 1993 November; 15(11):950-8)) and Pichia capsulata (DE10327454.4). Carbonyl reductase from Rhodococcus erythropolis (RECR) (U.S. Pat. No. 5,523,223), Norcardia fusca (Biosci. Biotechnol. Biochem., 63 (10) (1999), pp. 1721-1729), (Appl Microbiol Biotechnol. 2003 September; 62(4):380-6. Epub 2003 Apr. 26), and Rhodococcus ruber (J Org Chem. 2003 Jan. 24; 68(2):402-6.).
R-specific secondary alcohol dehydrogenases from organisms of the genus Lactobacillus (Lactobacillus kefir (U.S. Pat. No. 5,200,335), Lactobacillus brevis (DE 19610984 A1) (Acta Crystallogr D Biol Crystallogr. 2000 December; 56 Pt 12:1696-8), Lactobacillus minor (DE10119274) or Pseudomonas (U.S. Pat. No. 5,385,833) (Appl Microbiol Biotechnol. 2002 August; 59(4-5):483-7. Epub 2002 Jun. 26., J. Org. Chem. 1992, 57, 1532)
However, the enzymes known today are not nearly sufficient for exploiting the entire market potential of stereoselective reductions of keto compounds. On the one hand, this can be explained by the fact that the individual enzymes have very different properties with respect to substrate spectrum, pH optima as well as temperature and solvent stabilities, which often supplement each other. Therefore, even relatively similar homologous enzymes may exhibit a completely different conversion behaviour with regard to one particular substrate. On the other hand, not nearly all of the enzymes described are cloned and overexpressible to a sufficient extent, which means that these enzymes are not available for industrial use. For exploiting the synthetic potential of the enzymatic asymmetric hydrogenation as extensively as possible, it is therefore necessary to be in possession of a portfolio of different industrially accessible oxidoreductases which is as broad as possible, which oxidoreductases are furthermore suitable for use in aqueous/organic two-phase systems with organic solvents.